1. Field of the Invention
This invention relates in general to an optical pickup actuator (also referred to as objective-lens driving unit) for use on an optical disc drive to drive an objective lens used for focusing and tracking of a laser beam used to read data from an optical disc. More particularly, the invention relates to an optical pickup actuator with a magnetic-levitation structure which can allow the actuator to be restored to and then remain stably at its neutral position after a focusing and tracking operation is completed.
2. Description of Related Art
Optical discs are a mass storage medium that can store a large amount of digital data thereon, which can be read by using optical disc drives. An optical disc drive is provided with an optical pickup (read/write head) for reading the data stored on an optical disc. During read operation, the optical pickup should be capable of focusing a laser beam precisely on the target track where the data to be read out are located. An objective lens is used for the focusing purpose. A driving unit (or called actuator) is used to move the objective lens in various directions during the focusing and tracking operations to bring the laser beam into focused and tracked condition. Defocusing and mistracking of the laser beam are usually caused by deviations and oscillations of the optical disc being read. The actuator should have the following features: (1) high sensitivity; (2) good dynamic characteristics, such as the capability to maintain stable operation in high-frequency regions; and (3) compact in size and low in manufacturing cost.
Among conventional optical pickup actuators, the shaft/sleeve rotary type is the most widely used. During read operation, the actuator should be able to remain at the neutral position after the focusing and tracking operation is completed. The gravitation is the major factor that would pull the actuator away from the neutral position. In order to allow the actuator to remain at the neutral position, one conventional design scheme is to provide an elastic support on the actuator. One example of this design scheme is disclosed in the U.S. Pat. No. 4,792,935. One drawback to this design scheme, however, is that it would increase the structural complexity of the actuator. Another conventional design scheme is to provide a magnetic means to support the actuator at the neutral position. One example of this design scheme is disclosed in the U.S. Pat. No. 4,998,802 "OBJECTIVE LENS DRIVING DEVICE IN AN OPTICAL PICKUP" to Kasuga. In this patent, a pair of thin iron plates are fixedly mounted between two magnetic poles that can exert magnetic forces of equal magnitude and opposite directions on the thin iron plates to thereby maintain the actuator at the neutral position. One drawback to this design scheme, however, is that, the magnetic forces from the two magnetic poles can be easily unbalanced due to uneven magnetization on the magnetic poles. Should this be the case, the unbalanced magnetic forces acting on the thin iron plates will cause the actuator to be urged against its shaft axis, thus causing frictional force to the shaft that would degrade the performance of the actuator.